This invention relates to apparatus for changing the speed ratio between a first displaceable member and a second displaceable member in general, and more particularly to an improved apparatus of this nature for use in a recording instrument in which a dependent variable is recorded on a medium such as a strip chart with respect to an independent variable the scale of which may be varied within a predetermined range while maintaining a phase relationship between the first and second displaceable members that is correct for the scale in use.
There are various applications in which the speed ratio between first and second displaceable members must be selectively changed. Typically, the displaceable members where such is required are shafts disposed for rotation. In other words, there is normally an input shaft and an output shaft with means between to alter the speed ratio between the two. However, there are applications where altering speed ratios must be available between two rectilinearly displaceable members. Such may be the case where each displaceable member is a rectilinear stepper motor and a range of speed ratios is provided between the two by controlling the frequency ratio of the drive pulses to one and the other motor.
The need for a device of this nature which is capable of maintaining a known phase or angular relationship between the two members can best be understood with reference to an example, particularly an example with regard to a recording instrument of the type noted above. In particular, the need for such a device in a spectrophotometer incorporating a chart recorder will be described. In such a device, there is a need to have a range of selectable abscissa scales. An instrument of this nature imposes certain phasing requirements which cannot be met by conventional gearboxes and other similar apparatus for changing speeds. It exemplifies the problem which the present invention solves and is a type of device where the present invention may be put to extremely good use. Although such devices primarily require the ability to provide scale expansion on the chart recorder, in some cases scale compression is also necessary.
In a spectrophotometer, the abscissa normally represents the wavenumber scan. The wavenumber scan value at any time from the scan start is represented by a corresponding rotational angle of a shaft, which is herein referred to as the wavenumber shaft. In terms of the present introduction, this is the first displaceable member. The wavenumber shaft can be coupled to drive a recording pen bridge which travels relative to a stationary chart. Alternatively, the chart can be driven relative to a stationary pen bridge, in both cases to generate the abscissa. In fact, these two modes are mechanically equivalent and for the purpose of understanding the present invention it is only necessary to consider the one case. Implementation in the other manner will readily suggest itself to those skilled in the art. For the purpose of the present invention, the latter mode in which the chart is in the form of a roll of perforated paper driven by a sprocketed shaft will be considered. The sprocketed shaft, in the terms of the present introduction, is the second displaceable member. Hereinafter it will also be referred to as the recorder shaft.
In an instrument having no abscissa expansion, it is typical for the recorder shaft to be displaced by the wavenumber shaft of the instrument through a fixed ratio. A phasing problem arises if a conventional gear box which provides a range of speed ratios is to be used interposed between a wavenumber shaft and the recorder shaft. This results from the fact that no readily identifiable rotational relationship is maintained between the two shafts when, after interrupting the wavenumber scan at some intermediate value between the instrument scan limits, by throwing the gearbox into a "neutral" position, and selecting a new speed ratio, the scan is resumed.
A plain recording chart or one provided with a series of uniformly spaced uncalibrated abscissa grid lines would normally be used where the operator intended to make use of the abscissa expansion facilities, since it would not be practical to include on the same chart a calibration for each of the abscissa expansions provided. The loss of an interpretable rotational relationship between the two shafts would mean that if the operator, for example, wished to try different values of abscissa expansions from a given wavenumber value corresponding to a chosen datum position on the chart (say, a mark made by the operator or a given abscissa grid line) he would have to reset the start point after each run by reversing the wavenumber drive until said given wavenumber value came up on the wavenumber dial and then manually positioning the chart at said datum position - two tiresome tasks neither of which can be easily performed with good repeatability.
The practical result of an almost inevitable introduction of an inconstant abscissa shift from each resumption of scan following the selection of a new abscissa scale, whether in repetitively scanning the same region of the spectrum at different abscissa expansions or in expanding certain regions only in the course of a run, is that the operator is completely at sea when it comes to reading off with reasonable accuracy the wavenumber corresponding to a spectral feature of interest to him, because the abscissa scales lack any repeatable phasing with the wavenumber dial and no definite value related to the expansion factor can therefore be assigned to the abscissa grid lines and, consequently to any subdivisions marked between grid lines.
Where a calibrated chart was to be used in the uncalibrated mode to expand certain regions of the spectrum under observation, the operator would encounter a similar phasing difficulty in passing from the uncalibrated to the calibrated mode (and vice versa).
In view of these various difficulties the need for an improved apparatus for changing the ratio of speed between two displaceable members, particularly the need for a device for changing speed ratio between a shaft such as the wavenumber shaft and a second shaft, such as the recorder shaft, in an instrument such as a spectrophotometer which is provided with scale expansion capabilities, becomes evident.